The elephant in the room regarding HSS is that no matter what the shutter duration is, the curtain transit times are the same for a given camera. For the top tier DSLRs that's around 2.5 milliseconds, or 1/400 second. Entry level digital cameras used to be in the 4-5 ms (1/250-1/200) range but lately seem to be closer to around 3-4 ms. Everything else is in between. I know not how much faster, if any, the transit times will need to be for the 20 fps (mechanical shutter) cameras currently emerging in the market.
Keep in mind that for normal flash sync, both curtains must remain open long enough for flash signalling and flash duration to occur. So even if a camera has a transit time of 2.5ms (1/400), the second curtain can not begin to close immediately after the first curtain is fully open. It must wait until the "fire" signal has been sent and the flash has had time to release its energy, or at least 90% of it for a t.1 exposure. For speedlights, that's typically at least as long as the shutter curtain transit time. Speedlights are so named because they can reach T.1 so quickly. Many monolights tend to take much longer for a full power pulse to reach T.1 (mostly because they are releasing much more power than a typical speedlight), and that's why we often need to reduce Tv to well below the X-Sync for the camera to get the full benefit of a studio flash.
At 20 fps with a 2.5 ms shutter transit time in mirrorless/LV shooting there's still 47.5 ms less the exposure time (another 0.125ms for 1/8000, another 0.25ms for 1/4000, 0.5ms for 1/2000, 1 ms for 1/1000, and so on) available in each frame's 1/20 second (50ms) to reset the second shutter curtain which uncovers the sensor, do AF/metering, and then reset the first curtain to cover the sensor. With an OVF it probably gets a bit more difficult to drop the mirror, give it time to stop bouncing, do AF/metering, raise the mirror, give it time to stop bouncing, and confirm it is up in only 46-47.5 ms. If one looks at super slow-mo videos of DSLRs at 1/8000, it's pretty clear the shutter curtains wait until the mirror is almost all the way back down to begin resetting and they complete the reset before the mirror hits bottom and stops bouncing.
HSS is supposed to time the pulses of the flash so that as the slit between curtains moves across the sensor, each part of the sensor gets an equal number of pulses. Thus the timing of the pulses needs to be variable to accommodate different shutter speeds/slit widths or needs to be so fast that it is pulsing at least at the lowest common multiple of all of the available exposure times/slit widths in ms, which I imagine would be astronomically high if one assumes 1/3 stop Tv intervals are allowed.